This invention relates to a process for making necked nonwoven materials and laminates having edge regions with higher basis weight and increased extendibility relative to a central region, and to necked nonwoven materials produced by such process.
Necked nonwoven webs, including necked spunbond webs, meltblown webs, combinations and the like, and laminates including nonwoven webs such as spunbond film laminates, are often made using a process which is schematically illustrated in FIG. 1. A nonwoven web 12 having a starting width is passed in a machine direction between a first nip 16, which can be a first pair of nip rollers traveling at a first surface velocity, and a second nip 26, which can be a second pair of nip rollers traveling at a second surface velocity greater than the first surface velocity. The surface velocity difference between the first nip and the second nip results in formation of a necked or narrowed nonwoven web 22 having a necked width which is less than the starting width.
The starting nonwoven web 12 includes edge regions 13 and 15, and a central region 11. The necked nonwoven web 22 includes edge regions 23 and 25, and a central region 21. Because the necking causes the nonwoven fibers to become closer together and more aligned, without noticeably stretching or narrowing the individual fibers, the necked nonwoven web 22 generally has a higher basis weight than the starting nonwoven web 12.
As can be easily seen from FIG. 1, the nonwoven fibers in the edge regions 13 and 15 of the starting nonwoven web travel a greater distance between the first nip 16 and the second nip 26 during the necking process, than the fibers in the central region 11. Further, the cross-directional stresses in the central region 11 are at least partially counteracted, because these stresses are applied in both cross directions. The cross-directional stresses in each of the edge regions 13 and 15 are in one direction, inward toward the central region 11 of the nonwoven web. This results in increased fiber gathering and necking in the edge regions. Consequently, the fibers in the edge regions 23 and 25 of the necked nonwoven web are generally more aligned and closer together than the fibers in the central region 21. As a result, the necked nonwoven web becomes nonuniform in the cross direction, having greater gathering and thus a higher basis weight and extendibility in both edge regions than in the central region. If this necked web is then slit into a desired number of slits, the slits including each edge portion of the necked nonwoven web will have different properties, edge to edge, than the center slits.
There is a need or desire for a necking process which produces similar or identical slit necked nonwoven strips, each having a substantially similar cross-directional profile in basis weight and extendibility.
As used herein, the term xe2x80x9ccomprisingxe2x80x9d opens the claim to inclusion of additional materials and/or process steps other than those recited.
As used herein, the term xe2x80x9crecoverxe2x80x9d refers to a contraction of a stretched material upon termination of a biasing force following stretching of the material by application of the biasing force. For example, if a necked material having a relaxed, unbiased width of one (1) inch is elongated 50 percent in the cross direction by stretching to a width of one and one half (1.5) inches the material would be elongated 50 percent (0.5 inch) and would have a stretched width that is 150 percent of its relaxed width. If this exemplary stretched material is relaxed, and is recovered to a width of one and one tenth (1.1) inches after release of the biasing and stretching force, the material would have recovered 80 percent (0.4 inch) of its one-half (0.5) inch elongation. Recovery may be expressed as [(maximum stretched dimension minus final sample dimension)/(maximum stretched dimension minus initial sample dimension)]xc3x97100.
As used herein, the term xe2x80x9cnonwoven webxe2x80x9d means a web that has a structure of individual fibers of threads which are interlaid, but not in an identifiable repeating manner. Nonwoven webs have been, in the past, formed by a variety of processes such as, for example, spunbonding processes, meltblowing processes and bonded carded web processes.
As used herein, the term xe2x80x9cmicrofibersxe2x80x9d means small diameter fibers having an average diameter not greater than about 75 microns, for example, having a diameter of from about 0.5 microns to about 75 microns, more specifically microfibers may also have an average diameter of from of from about 4 microns to about 40 microns.
As used in herein, the term xe2x80x9cinterfiber bondingxe2x80x9d means bonding produced by thermal bonding or entanglement between the individual nonwoven fibers to form a coherent web structure. Fiber contact bonding and entangling are inherent in the meltblown processes but may be generated or increased by processes such as, for example, hydraulic entangling or needle punching. One or more thermal bonding steps are employed in most processes for forming spunbond webs. Alternatively or additionally, a bonding agent can be utilized to increase the desired bonding and to maintain structural coherency of the web. For example, powdered bonding agents and chemical solvent bonding may be used.
As used herein, the term xe2x80x9cmeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into a high velocity gas (e.g. air) stream which attenuates the filaments of molten thermoplastic material to reduce their diameters, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin, the disclosure of which is hereby incorporated by reference.
As used herein, the term xe2x80x9cspunbonded fibersxe2x80x9d refers to small diameter fibers which are formed by extruding a molten thermoplastic material as filaments from plurality of fine, usually circular, capillaries in a spinneret with the diameter of the extruded filaments then being rapidly reduced, for example, by eductive drawing or other well-known spun bonding mechanisms. The production of spun-bonded nonwoven webs is illustrated in patents such as, for example, in U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al. The disclosures of these patents are hereby incorporated by reference.
xe2x80x9cNeckedxe2x80x9d or xe2x80x9cneck stretchedxe2x80x9d are interchangeable terms and refer to a method of elongating a nonwoven fabric, generally in the longitudinal, or machine direction, to reduce its width in a controlled manner to a desired amount. The controlled stretching may take place under cool, room temperature or greater temperatures and is limited to an increase in overall dimension in the direction being stretched up to the elongation required to break the fabric, which in many cases is about 1.05 to 1.7 times. When relaxed, the web returns toward its original dimensions. Such a process is disclosed, for example, in U.S. Pat. Nos. 4,443,513 to Meitner and Notheis; U.S. Pat. Nos. 4,965,122, 4,981,747 and 5,114,781 to Morman; and U.S. Pat. No. 5,244,482 to Hassenboehler Jr. et al.
As used herein, the term xe2x80x9cnecked materialxe2x80x9d refers to any material which has been constricted in at least one dimension by processes such as, for example, drawing.
As used herein, the term xe2x80x9cneckable materialxe2x80x9d means any material which can be necked.
As used herein, the term xe2x80x9creversibly necked materialxe2x80x9d refers to a necked material that has been treated while necked to impart memory to the material so that, when a force is applied to extend the material to its prenecked dimensions, the necked and treated portions will generally recover to their necked dimensions upon termination of the force. One form of treatment is the application of heat. Generally, extension of the reversibly necked material is substantially limited to extension to its prenecked dimensions. Therefore, unless the material is elastic, extension too far beyond its prenecked dimensions will result in material failure. A reversibly necked material may include more than one layer, for example, multiple layers of spunbond web, multiple layers of meltblown web, multiple layers of bonded carded web or any other suitable combination or mixtures thereof, including laminates containing a film and/or a foam, as described for example in U.S. Pat. No. 4,965,122, the disclosure of which is incorporated herein by reference.
As used herein, the term xe2x80x9cpolymerxe2x80x9d generally includes, but is not limited to homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Further, unless otherwise specifically limited, the term xe2x80x9cpolymerxe2x80x9d shall include all possible geometrical configurations of the material. These configurations include, but are not limited to, isotactic, syndiotactic and random symmetries.
The present invention is directed to a process for making a necked material, for example a necked nonwoven web having better cross-directional basis weight uniformity and extendibility uniformity strip to strip, and to necked nonwoven webs produced by such process. A nonwoven web, for example a polypropylene spunbond web, which may be unwound from a roll or provided directly from a spunbond process, is pulled in a machine direction by a first pair of counter rotating nip rollers and a second pair of counter rotating nip rollers. In certain embodiments of this invention, the polypropylene spunbond web may be formed from a polypropylene homopolymer, or a random propylene-ethylene copolymer containing up to 10% by weight ethylene and at least 90% by weight propylene. As the nonwoven web moves in the machine direction, it is slit in the machine direction by a plurality of knives or other slitting devices, causing formation of a plurality of nonwoven strips.
The nonwoven strips may pass straight between the first nip rollers or may pass between the first nip rollers in another configuration, such as an S-wrap fashion. The nonwoven strips can also wrap around rolls with surfaces having a higher coefficient of friction to put tension on the strips and draw the strips. The drawing tension can also result from pulling the material directly off a supply roll and controlling the speed of the unwinding roll. After passing between the first nip rollers, the nonwoven strips enter a necking zone, defined as the distance between the first nip rollers and a second pair of nip rollers. A through-air heating oven or another suitable heating device may be provided in the necking zone to heat the nonwoven strips. A suitable temperature for a spunbond web is about 180xc2x0 F. (82xc2x0 C.) to about 280xc2x0 F. (138xc2x0 C.) or about 85xc2x0 C. to about 30xc2x0 C. below its melting point. The supplied heat increases the degree of necking before breaking and may also cause the necked nonwoven strips to become heat set (reversibly necked). Nonwoven setting can also occur by ageing freshly spun fibers in a necked configuration on a slit roll.
The second nip rollers turn at a surface velocity greater than the first nip rollers, suitably about 5% to about 40% greater than the first nip rollers. The increased speed of the second nip rollers relative to the first nip rollers and the heat applied to the nonwoven strips in the necking zone cause each of the nonwoven strips to become necked or narrowed from an initial or starting width to a necked width which is less than the initial width, suitably about 20% to about 80% of the initial width, resulting in the formation of necked nonwoven strips. The second nip rollers can have an outer surface with a higher coefficient of friction to put tension on the strips and draw the strips. The necked nonwoven strips may each have a necked width of at least about 2 inches, and may be substantially wider depending on the initial width of the nonwoven web, the number of nonwoven strips formed in the nonwoven web and the amount of necking.
The necking causes the filaments in each nonwoven strip to become more machine-direction aligned and compact, and does not cause significant elongation of individual nonwoven filaments. As a result, each necked nonwoven strip has a higher basis weight than its basis weight prior to necking. By slitting the nonwoven web into narrower strips, the resulting adjacent necked nonwoven strips are similar or identical, each strip having a similar cross-sectional profile in both basis weight and extendibility. Each necked nonwoven strip can be easily stretched in the cross direction by between about 25% to about 500% of its necked width. If the necked nonwoven strip is heat set (reversibly necked) it may then recover toward its necked width upon release of the stretching force.
The features and advantages of the invention will become further apparent from the following detailed description of the invention, read in conjunction with the accompanying drawings. The detailed description and drawings are intended to be illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.